Ask access(X_OK) whether an overlay PATH match may run, on every target [patch]

RunCommand.Test/RunCommandTests.cs

@@ -934,6 +934,56 @@ public async Task ExecuteAsyncShouldSkipANonExecutableFileOnTheOverlayPath()
 		Assert.AreSequenceEqual(ExpectedOverlayCopy, output, "Expected the search to pass over a file without an execute bit, as a shell does.");
 	}
 
+	[TestMethod]
+	public async Task ExecuteAsyncShouldSkipAFileOnTheOverlayPathThatOnlyOthersMayExecute()
+	{
+		SkipOnWindows();
+		if (Environment.UserName == "root")
+		{
+			Assert.Inconclusive("root may execute any file that has an execute bit, so the owner's missing bit is not visible.");
+		}
+
+		// Group and others may execute this file but its owner, the caller, may not, so a shell passes over it.
+		// Testing for any execute bit accepted it and the start failed with "Permission denied" (#139).
+		string root = CreateDirectoryForTest();
+		string othersOnly = Directory.CreateDirectory(Path.Join(root, "a")).FullName;
+		string executable = Directory.CreateDirectory(Path.Join(root, "b")).FullName;
+		string name = $"runcommand-{nameof(ExecuteAsyncShouldSkipAFileOnTheOverlayPathThatOnlyOthersMayExecute)}";
+		WriteScript(othersOnly, name, "echo ran the file the caller cannot run", executable: false);
+		if (!OperatingSystem.IsWindows())
+		{
+			File.SetUnixFileMode(
+				Path.Join(othersOnly, name),
+				UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.GroupRead | UnixFileMode.GroupExecute | UnixFileMode.OtherRead | UnixFileMode.OtherExecute);
+		}
+
+		WriteScript(executable, name, "echo ran overlay copy", executable: true);
+
+		List<string> output = await RunOnOverlayPathAsync(name, $"{othersOnly}{Path.PathSeparator}{executable}").ConfigureAwait(false);
+
+		Assert.AreSequenceEqual(ExpectedOverlayCopy, output, "Expected the search to ask whether the caller may execute the file, as a shell does.");
+	}
+
+	[TestMethod]
+	public void CanExecuteShouldReportWhetherTheCallerMayExecuteTheFile()
+	{
+		SkipOnWindows();
+		string directory = CreateDirectoryForTest();
+		WriteScript(directory, "runs", "exit 0", executable: true);
+		WriteScript(directory, "does-not-run", "exit 0", executable: false);
+
+		Assert.IsTrue(NativeMethods.CanExecute(Path.Join(directory, "runs")));
+		Assert.IsFalse(NativeMethods.CanExecute(Path.Join(directory, "does-not-run")));
+	}
+
+	[TestMethod]
+	public void CanExecuteShouldKeepTheFileWhenTheCLibraryCannotBeCalled()
+	{
+		// Without access(), the search keeps every existing file, as it did before it checked permissions.
+		Assert.IsTrue(NativeMethods.CanExecute("/any/file", (_, _) => throw new DllNotFoundException("libc")));
+		Assert.IsTrue(NativeMethods.CanExecute("/any/file", (_, _) => throw new EntryPointNotFoundException("access")));
+	}
+
 	[TestMethod]
 	public async Task ExecuteAsyncShouldFindTheToolOnTheCallersPathWhenTheOverlayRemovesPath()
 	{

RunCommand/NativeMethods.cs

@@ -0,0 +1,52 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.RunCommand;
+
+using System;
+using System.Runtime.InteropServices;
+
+/// <summary>
+/// Calls into the C library on Linux and macOS.
+/// </summary>
+internal static class NativeMethods
+{
+	/// <summary>The <c>X_OK</c> mode of <c>access</c>: test for permission to execute.</summary>
+	private const int ExecuteOk = 1;
+
+	/// <summary>
+	/// Returns whether the calling process may execute the file at <paramref name="path"/>.
+	/// </summary>
+	/// <param name="path">The full path of an existing file.</param>
+	/// <returns>
+	/// <see langword="true"/> when <c>access(path, X_OK)</c> succeeds, or when the C library cannot be called, which
+	/// keeps the file as a candidate the way the search did before it checked permissions.
+	/// </returns>
+	/// <remarks>
+	/// <c>access</c> answers for the caller, as a shell's <c>PATH</c> search does. Testing for any execute bit instead
+	/// would accept a <c>0700</c> file another user owns, and <c>File.GetUnixFileMode</c> does not exist below .NET 7,
+	/// so the net5.0, net6.0 and netstandard builds had no check at all (ktsu-dev/RunCommand#139).
+	/// </remarks>
+	internal static bool CanExecute(string path) => CanExecute(path, Access);
+
+	/// <summary>
+	/// Returns whether <paramref name="access"/> reports that the calling process may execute <paramref name="path"/>.
+	/// </summary>
+	/// <param name="path">The full path of an existing file.</param>
+	/// <param name="access">Performs <c>access(path, mode)</c>. A parameter so tests can make the C library unavailable.</param>
+	/// <returns>As <see cref="CanExecute(string)"/>.</returns>
+	internal static bool CanExecute(string path, Func<string, int, int> access)
+	{
+		try
+		{
+			return access(path, ExecuteOk) == 0;
+		}
+		catch (Exception exception) when (exception is DllNotFoundException or EntryPointNotFoundException)
+		{
+			return true;
+		}
+	}
+
+	[DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+	[DllImport("libc", EntryPoint = "access", CharSet = CharSet.Ansi, BestFitMapping = false, ThrowOnUnmappableChar = true)]
+	private static extern int Access(string path, int mode);
+}

RunCommand/RunCommand.cs

@@ -24,7 +24,7 @@
 	/// </summary>
 	/// <param name="command">The command to execute.</param>
 	/// <returns>The exit code of the executed process.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static int Execute(string command) =>
 		ExecuteAsync(command).Result;
@@ -35,7 +35,7 @@
 	/// <param name="command">The command to execute.</param>
 	/// <param name="outputHandler">The handler for processing command output.</param>
 	/// <returns>The exit code of the executed process.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static int Execute(string command, OutputHandler outputHandler) =>
 		ExecuteAsync(command, outputHandler).Result;
@@ -46,7 +46,7 @@
 	/// <param name="command">The command to execute.</param>
 	/// <param name="elevation">The privilege level under which to run the command.</param>
 	/// <returns>The exit code of the executed process.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static int Execute(string command, Elevation elevation) =>
 		ExecuteAsync(command, elevation).Result;
@@ -62,7 +62,7 @@
 	/// </param>
 	/// <param name="elevation">The privilege level under which to run the command.</param>
 	/// <returns>The exit code of the executed process.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static int Execute(string command, OutputHandler outputHandler, Elevation elevation) =>
 		ExecuteAsync(command, outputHandler, elevation).Result;
@@ -109,7 +109,7 @@
 	/// </summary>
 	/// <param name="command">The command to execute.</param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command)
 		=> await ExecuteAsync(command, new OutputHandler()).ConfigureAwait(false);
@@ -120,7 +120,7 @@
 	/// <param name="command">The command to execute.</param>
 	/// <param name="outputHandler">The handler for processing command output.</param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command, OutputHandler outputHandler)
 		=> await ExecuteAsync(command, outputHandler, Elevation.Default).ConfigureAwait(false);
@@ -131,7 +131,7 @@
 	/// <param name="command">The command to execute.</param>
 	/// <param name="elevation">The privilege level under which to run the command.</param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command, Elevation elevation)
 		=> await ExecuteAsync(command, new(), elevation).ConfigureAwait(false);
@@ -147,7 +147,7 @@
 	/// </param>
 	/// <param name="elevation">The privilege level under which to run the command.</param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command, OutputHandler outputHandler, Elevation elevation)
 		=> await ExecuteAsync(command, outputHandler, elevation, CancellationToken.None).ConfigureAwait(false);
@@ -160,7 +160,7 @@
 	/// A token that, when cancelled, terminates the running process and its children.
 	/// </param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command, CancellationToken cancellationToken)
 		=> await ExecuteAsync(command, new OutputHandler(), cancellationToken).ConfigureAwait(false);
@@ -175,7 +175,7 @@
 	/// A token that, when cancelled, terminates the running process and its children.
 	/// </param>
 	/// <returns>A task representing the asynchronous operation with the process exit code.</returns>
 	[Obsolete("A command string is split on its first space, which cannot handle an executable path "
 		+ "containing spaces. Use the overload taking a file name and an argument list instead.")]
 	public static async Task<int> ExecuteAsync(string command, OutputHandler outputHandler, CancellationToken cancellationToken)
 		=> await ExecuteAsync(command, outputHandler, Elevation.Default, cancellationToken).ConfigureAwait(false);
@@ -504,8 +504,8 @@
 	/// <param name="directory">A directory from the search path.</param>
 	/// <param name="fileName">The file name to look for.</param>
 	/// <returns>
-	/// The full path, or <see langword="null"/> if there is no such file. Outside Windows the file also needs an
-	/// execute permission bit, where the platform exposes one.
+	/// The full path, or <see langword="null"/> if there is no such file. Outside Windows the calling process also
+	/// needs permission to execute the file, as <c>access(X_OK)</c> reports it, on every target framework.
 	/// </returns>
 	private static string? TryExecutable(string directory, string fileName)
 	{
@@ -524,13 +524,10 @@
 				return null;
 			}
 
-#if NET7_0_OR_GREATER
-			const UnixFileMode anyExecute = UnixFileMode.UserExecute | UnixFileMode.GroupExecute | UnixFileMode.OtherExecute;
-			if (!OperatingSystem.IsWindows() && (File.GetUnixFileMode(candidate) & anyExecute) == 0)
+			if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && !NativeMethods.CanExecute(candidate))
 			{
 				return null;
 			}
-#endif
 
 			return candidate;
 		}